The enzyme, carbonic anhydrase (“CA”) (EC 4.2.1.1), catalyzes the reversible reactions depicted in Scheme 1:

In the forward or “hydration” reaction, CA combines carbon dioxide and water to provide bicarbonate and a proton, or depending on the pH, to provide carbonate (CO3−2) and two protons. In the reverse, or “dehydration” reaction, CA combines bicarbonate and a proton to provide carbon dioxide and water. Carbonic anhydrases are metalloenzymes that typically have Zn+2 in the active site. However carbonic anhydrases having e.g. Co+2 or Cd+2 in the active site have been reported. At least three classes of carbonic anhydrases have been identified in nature.
The α-class carbonic anhydrases are found in vertebrates, bacteria, algae, and the cytoplasm of green plants. Vertebrate α-carbonic anhydrases are among the fastest enzymes known, exhibiting a turnover number (kcat) (the number of molecules of substrate converted by an enzyme to product per catalytic site per unit of time) of 106 sec−1. The β-class carbonic anhydrases are found in bacteria, algae, and chloroplasts, while γ-class carbonic anhydrases are found in Archaea and some bacteria. Although carbonic anhydrases of each of these classes have similar active sites, they do not exhibit significant overall amino acid sequence homology and they are structurally distinguishable from one another. Hence, these three classes of carbonic anhydrase provide an example of convergent evolution.
It has been proposed that carbonic anhydrases could be used as a biological catalyst to accelerate the capture of carbon dioxide produced by produced by combustion of fossil fuels. See e.g., U.S. Pat. Nos. 6,143,556, 6,524,843 B2, 7,176,017 B2, 7,596,952 B2, 7,579,185 B2, 7,740,689 B2, 7,132,090 B2; U.S. Pat. Publ. Nos. 2009/0155889A1, 2010/0086983A1; PCT Publ. Nos. WO2006/089423A1, WO2010/014773A1, WO2010/045689A1. State-of-the-art carbonic anhydrases, however, are not well-suited for use in such applications because of their relative lack of stability and/or activity under the process conditions required. Accordingly, there is a need in the art for carbonic anhydrases that can effectively accelerate the absorption of carbon dioxide from a gas stream and/or later accelerate desorption of the carbon dioxide from the capture solution under process relevant conditions, such as presence in solution with high concentrations of other CO2 absorption mediating compounds (e.g., amines, ammonia, carbonate ions), elevated temperatures (e.g., 40° C. or above, or 15° C. or below in NH3), alkaline pHs (e.g., pH 8-12), and extended periods of exposure to these challenging conditions (e.g., days to weeks). In addition, such carbonic anhydrases should also be stable to variations in these conditions, e.g. stable not only at a relatively alkaline pH suitable for hydration and sequestration of carbon dioxide but also at a relatively acidic pH suitable for subsequent release and/or recapture of the hydrated and/or sequestered carbon dioxide.